Imaging apparatus including light source that emits pulsed light, image sensor, and control circuit

ABSTRACT

An imaging apparatus including a light source that, in operation, emits pulsed light to a measurement target; a diffusion member that is disposed between the light source and the measurement target, and diffuses the pulsed light; an image sensor that includes at least one pixel, the at least one pixel including a photodiode and a charge accumulator that, in operation, accumulates signal charge from the photodiode; and a control circuit that, in operation, controls the image sensor. The control circuit causes the image sensor to start to accumulate the signal charge with the charge accumulator in a falling period of a returned pulsed light which is returned from the measurement target to the image sensor due to the emission of the pulsed light, the falling period being a period from start to end of a decrease of an intensity of the returned pulsed light.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is the Continuation of U.S. patent application Ser. No.15/175,340, filed on Jun. 7, 2016, which in turn claims the benefit ofJapanese Application No. 2015-133892, filed on Jul. 2, 2015 and JapanesePatent Application No. 2015-122390, filed on Jun. 17, 2015, the entiredisclosures of which Applications are incorporated by reference herein.

BACKGROUND 1. Technical Field

The present disclosure relates to an imaging apparatus.

2. Description of the Related Art

In fields of biometrics and material analyses, methods are used in whichlight is emitted to a target and internal information regarding thetarget is obtained on the basis of information regarding the light thathas passed through the target. In these methods, components reflectedfrom a surface of the target sometimes act as noise. As a method forremoving noise caused by these surface reflection components andobtaining only desired internal information, a method disclosed inJapanese Unexamined Patent Application Publication No. 11-164826, forexample, is known in the field of biometrics. In Japanese UnexaminedPatent Application Publication No. 11-164826, a method is disclosed inwhich a light source and a photodetector are attached to a target withthe light source and the photodetector separated from each other by acertain distance.

SUMMARY

In one general aspect, the techniques disclosed here feature an imagingapparatus including a light source that, in operation, emits pulsedlight to a measurement target; a diffusion member that is disposedbetween the light source and the measurement target, and diffuses thepulsed light; an image sensor that includes at least one pixel, the atleast one pixel including a photodiode and a charge accumulator that, inoperation, accumulates signal charge from the photodiode; and a controlcircuit that, in operation, controls the image sensor, wherein thecontrol circuit, in operation, causes the image sensor to start toaccumulate the signal charge with the charge accumulator in a fallingperiod of a returned pulsed light which is returned from the measurementtarget to the image sensor due to the emission of the pulsed light, thefalling period being a period from start to end of a decrease of anintensity of the returned pulsed light.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram illustrating an imaging apparatusaccording to a first embodiment and a state in which the imagingapparatus captures an image of a measurement target;

FIG. 1B is a diagram illustrating the pulse width dependence of timecharacteristics of the amount of light detected by a sensor;

FIG. 1C is a diagram illustrating the pulse width dependence of theamount of light detected by the sensor;

FIG. 1D is a diagram illustrating an example of a schematicconfiguration of a pixel of an image sensor;

FIG. 1E is a diagram illustrating an example of the configuration of theimage sensor;

FIG. 1F is a diagram illustrating emission of first pulsed light andsecond pulsed light in one frame;

FIG. 1G is a flowchart schematically illustrating an operation performedby a control circuit;

FIG. 2 is a diagram illustrating optical signals caused by rectangularpulsed light that has been emitted from a light source, reflected fromthe measurement target, and has reached the image sensor;

FIG. 3A is a diagram illustrating a relationship between an opticalsignal in the image sensor, a shutter timing, and a detected opticalsignal according to a second embodiment;

FIG. 3B is a diagram illustrating a relationship between the pulsedlight emitted from the light source, the optical signal in the imagesensor, and the shutter timing according to the second embodiment;

FIG. 4A is a flowchart illustrating an operation performed by an imagingapparatus according to the second embodiment;

FIG. 4B is a flowchart illustrating an operation performed by theimaging apparatus according to an embodiment different from thatillustrated in FIG. 4A;

FIG. 5 is a schematic diagram illustrating an imaging apparatusaccording to a third embodiment and a state in which the imagingapparatus captures the measurement target;

FIG. 6A is a diagram illustrating an example of measurement of changesin brain activity according to a fourth embodiment using an imagingapparatus in the present disclosure;

FIG. 6B is a diagram illustrating another example of the measurement ofchanges in brain activity according to the fourth embodiment using theimaging apparatus in the present disclosure;

FIG. 7A is a diagram illustrating an emission area of light;

FIG. 7B is a diagram illustrating a two-dimensional brain activitydistribution in a detection area;

FIG. 8A is a diagram illustrating an example in which a materialanalysis and a structure analysis are performed on an inside of themeasurement target according to a fifth embodiment using the imagingapparatus in the present disclosure;

FIG. 8B is a diagram illustrating another example in which a materialanalysis and a structure analysis are performed on the inside of themeasurement target according to the fifth embodiment using the imagingapparatus in the present disclosure;

FIG. 9A is a diagram illustrating an example in which a materialanalysis and a structure analysis are performed on the inside of themeasurement target by separating an internal scattering component from asurface reflection component in light reflected from the measurementtarget according to the fifth embodiment using the imaging apparatus inthe present disclosure;

FIG. 9B is a diagram illustrating an example in which a materialanalysis and a structure analysis are performed on the inside of themeasurement target by obtaining multi-wavelength information regardingthe internal scattering component according to the fifth embodimentusing the imaging apparatus in the present disclosure;

FIG. 10 is a diagram illustrating the configuration of time resolutionof the internal scattering component according to a sixth embodiment;

FIG. 11 is a diagram illustrating a configuration according to a seventhembodiment by which the internal scattering component that has reached aposition relatively distant from a surface of the measurement target isdetected;

FIG. 12 is a diagram illustrating a result of measurement in a firstexample; and

FIG. 13 is a diagram illustrating a result of measurement of changes inbrain activity in the first example.

DETAILED DESCRIPTION

Before describing embodiments of the present disclosure, a result of anexamination of a method described in Japanese Unexamined PatentApplication Publication No. 4-189349, which is an example of the relatedart that measures internal information regarding a target in anoncontact manner, will be described hereinafter.

In Japanese Unexamined Patent Application Publication No. 4-189349,information located at different positions in a depth direction in aninside of the target is distinguished from each other through timeresolution. Light that has been emitted from a light source and haspassed through the target reaches a photodetector later as the lightreaches deeper into the target. The information in the depth directionis distinguished on the basis of this time difference. In JapaneseUnexamined Patent Application Publication No. 4-189349, ultrashortpulsed light whose pulse width is hundreds of femtoseconds to severalpicoseconds is emitted to the target in order to perform measurementwith a spatial resolution of 2 to 3 mm in the depth direction. Inaddition, a streak camera having almost the same (about a value obtainedby dividing 2 to 3 mm by light speed) temporal resolution detects thelight. The streak camera converts the light that has reached a lightreceiving unit (photoelectric conversion unit) thereof into electronsand sweeps the electrons at high speed in a direction perpendicular to atraveling direction. As a result, spatial displacements occur inaccordance with times at which photons have reached the light receivingunit. By detecting these displacements using a two-dimensionalfluorescent screen, temporal information can be converted into spatialinformation.

According to the examination conducted by the present inventors, in themethod described in Japanese Unexamined Patent Application PublicationNo. 4-189349, measurement is performed using a two-dimensionalfluorescent screen, but one of the two dimensions is used for thetemporal information. That is, the spatial information regarding thetarget is obtained using only the remaining one dimension. In addition,since an ultrashort pulsed light source whose pulse width is hundreds offemtoseconds to several picoseconds and a streak camera is used, cost isundesirably extremely high.

An imaging apparatus according to a first aspect of the presentdisclosure includes a light source that, in operation, emits pulsedlight to a living body, an image sensor that includes at least one pixelincluding a photodiode and charge accumulators that, in operation,accumulate signal charge from the photodiode, and a control circuitthat, in operation, controls the image sensor, wherein the chargeaccumulators, in operation, accumulate the signal charge correspondingto a component of the pulsed light scattered inside the living body.

An imaging apparatus according a second aspect of the present disclosureis the imaging apparatus according to the first aspect further includingan arithmetic circuit that, in operation, obtains biological informationregarding the living body by calculating the signal charge correspondingto the component of the pulsed light scattered inside the living body.

The biological information may be information regarding a cerebral bloodflow.

An imaging apparatus according to a third aspect of the presentdisclosure is the imaging apparatus according to the first or secondaspect in which: the image sensor further includes an electronicshutter; the control circuit, in operation, causes the electronicshutter to prevent the charge accumulators from accumulating the signalcharge while a part of the pulsed light is reflected from a surface ofthe living body and reaches the image sensor; and the control circuit,in operation, causes the electronic shutter to allow the chargeaccumulators to accumulate the signal charge while another part of thepulsed light is scattered inside the living body and reaches the imagesensor.

An imaging apparatus according to a fourth aspect of the presentdisclosure is the imaging apparatus according to any of the first tothird aspects in which the at least one pixel comprises pixels arrangedin two dimensions.

An imaging apparatus according to a fifth aspect of the presentdisclosure is the imaging apparatus according to the third aspect inwhich the control circuit, in operation, causes the electronic shutterto allow the charge accumulators to begin to accumulate the signalcharge when a trailing edge of the pulsed light is reflected from thesurface of the living body and reaches the image sensor or later.

An imaging apparatus according to a sixth aspect of the presentdisclosure is the imaging apparatus according to the first or secondaspect in which: the control circuit, in operation, causes the chargeaccumulators to begin to accumulate the signal charge a period of timeafter the light source begins to emit the pulsed light; and the controlcircuit, in operation, determines the period of time on the basis ofintensity of the signal charge accumulated in the charge accumulators.

An imaging apparatus according to a seventh aspect of the presentdisclosure is the imaging apparatus according to the first or secondaspect in which: the control circuit, in operation, causes the chargeaccumulators to begin to accumulate the signal charge a period of timeafter the light source begins to emit the pulsed light; the controlcircuit, in operation, calculates a distance between the image sensorand the living body; and the control circuit, in operation, determinesthe period of time on the basis of the distance.

An imaging apparatus according to an eighth aspect of the presentdisclosure is the imaging apparatus according to any of the first toseventh aspects in which the light source, in operation, emits firstpulsed light and second pulsed light in a wavelength band different froma wavelength band of the first pulsed light.

An imaging apparatus according to a ninth aspect of the presentdisclosure is the imaging apparatus according to any of the first toeighth aspects in which the image sensor is configured to obtain amulti-wavelength image.

An imaging apparatus according to a tenth aspect of the presentdisclosure is the imaging apparatus according to the first or secondaspect in which: the control circuit, in operation, causes the chargeaccumulators to accumulate the signal charge at a plurality of timeswhen a trailing edge of the pulsed light is reflected from a surface ofthe living body and reaches the image sensor or later; and the controlcircuit, in operation, obtains an optical length distribution of thecomponent of the pulsed light scattered inside the living body on thebasis of a change in intensity of the signal charge accumulated at theplurality of times.

An imaging apparatus according to an eleventh aspect of the presentdisclosure is the imaging apparatus according to any of the first totenth aspects further including a correction circuit that, in operation,corrects movement of the living body.

An imaging apparatus according to a twelfth aspect of the presentdisclosure is the imaging apparatus according to the eleventh aspect inwhich the correction circuit, in operation, corrects the movement of theliving body by detecting periodic vibration of the living body.

The embodiments that will be described hereinafter are general orspecific examples. Values, shapes, materials, components, positions atwhich the components are arranged, and the like described in thefollowing embodiments are examples, and do not limit the presentdisclosure. Among the components described in the following embodiments,ones not described in an independent claim, which defines a broadestconcept, will be described as arbitrary components.

The embodiments will be specifically described hereinafter withreference to the drawings.

First Embodiment 1. Imaging Apparatus

First, the configuration of an imaging apparatus D1 according to a firstembodiment will be described with reference to FIGS. 1A to 2.

FIG. 1A is a schematic diagram illustrating the imaging apparatus D1according to the present embodiment. The imaging apparatus D1 includes alight source Ls, an image sensor S, and a control circuit Sy.

1-1. Light Source Ls

The light source Ls emits light to a measurement target O. The lightthat has been emitted from the light source Ls and has reached themeasurement target O is divided into a component (surface reflectioncomponent I1) reflected from a surface of the measurement target O and acomponent (internal scattering component I2) reflected or scattered onceor scattered multiple times inside the measurement target O. The surfacereflection component I1 includes a direct reflection component, adiffuse reflection component, and a scattering reflection component. Thedirect reflection component is a reflection component whose incidentangle and reflection angle are the same. The diffuse reflectioncomponent is a component diffusely reflected from uneven portions of thesurface. The scattering reflection component is a component scattered byinternal tissues near the surface. If the measurement target O is humanskin, the scattering reflection component is a component scatteredinside epidermis. In the present disclosure, the component (surfacereflection component I1) reflected from the surface of the measurementtarget O will be described as including these three components. Theinternal scattering component I2 that will be described hereinafter,therefore, does not include the component scattered by the internaltissues near the surface. Traveling directions of the surface reflectioncomponent I1 and the internal scattering component I2 change due toreflection or scattering, and part of the surface reflection componentI1 and the internal scattering component I2 reaches the image sensor S.The light source Ls generates pulsed light a plurality of times atcertain time intervals or at certain timings. A fall time of the pulsedlight generated by the light source Ls may be close to zero and, forexample, the pulsed light is a rectangular wave. A rise time of thepulsed light generated by the light source Ls may be arbitrarilydetermined. This is because in measurement in which an imaging apparatusin the present disclosure, which will be described later, is used, afalling edge of pulsed light along a time axis is used, but a risingedge is not used. The light source Ls is, for example, a laser such as alaser diode (LD) whose pulsed light includes a falling edgeapproximately perpendicular to the time axis (having rapid time responsecharacteristics).

If the measurement target O is a living body, for example, wavelengthsachieved by the light source Ls may be set between about 650 nm andabout 950 nm. The wavelength range is included in a wavelength range ofred light to near-infrared light. The term “light” herein is used notonly for visible light but also for infrared light.

In order to measure the measurement target O in a noncontact manner, theimaging apparatus D1 in the present disclosure takes into considerationan effect upon the retina if the measurement target O is a person. Forthis reason, the imaging apparatus D1 may satisfy Class 1 of a lasersafety standard set by each country. In this case, light whose intensityis so low that an accessible emission limit (AEL) falls below 1 mW isemitted to the measurement target O. The light source Ls itself,however, need not satisfy Class 1. Class 1 of the laser safety standardmay be satisfied, for example, by providing a diffusion plate, a neutraldensity (ND) filter, or the like in front of the light source Ls anddiffusing or attenuating light.

The conventional streak camera described in Japanese Unexamined PatentApplication Publication No. 4-189349 has been used for distinguishinginformation (e.g., absorption coefficients or scattering coefficients)located at different positions in a depth direction of a living bodyfrom each other. In order to perform measurement with a desired spatialresolution, therefore, ultrashort pulsed light whose pulse width is onthe order of femtoseconds or picoseconds has been used. On the otherhand, the imaging apparatus D1 in the present disclosure is used fordistinguishing the internal scattering component I2 from the surfacereflection component I1. The pulsed light emitted by the light sourceLs, therefore, need not be ultrashort pulsed light, that is, the pulsewidth may be arbitrarily determined. When light is emitted to theforehead in order to measure a cerebral blood flow, the amount of lightof the internal scattering component I2 is extremely smaller than thatof the surface reflection component I1, namely one thousandth to oneten-thousandth of that of the surface reflection component I1.Furthermore, if the laser safety standard is taken into consideration,the amount of light that may be emitted becomes small, and it becomesvery difficult to detect the internal scattering component I2. Theamount of light detected can be increased and a signal-to-noise (SN)ratio can be improved, for example, if the light source Ls generatespulsed light whose pulse width is relatively large.

The light source Ls, for example, emits pulsed light having a pulsewidth of 3 ns or more. The temporal extension of light scattered insidea living body such as a brain is generally about 4 ns. As illustrated inFIG. 1B, therefore, as the width of the pulsed light emitted by thelight source Ls increases, the amount of light of the internalscattering component I2 that appears at a trailing edge of the pulsedlight returning from the measurement target O increases. FIG. 1C is adiagram in which a horizontal axis represents the width of input pulsedlight and a vertical axis represents the amount of light detected by theimage sensor S. The image sensor S includes an electronic shutter.Because a ratio of the surface reflection component I1 to the internalscattering component I2 is high immediately after a trailing edge of thepulsed light, the electronic shutter is opened 1 ns after the trailingedge. According to FIG. 1C, when the pulse width of the pulsed lightemitted by the light source Ls is 3 ns or more, the amount of lightdetected by the image sensor S can be maximized.

The light source Ls may emit pulsed light having a pulse width of 5 nsor more, or 10 ns or more. On the other hand, when the pulse width istoo large, the amount of light wasted increases. The light source Ls,therefore, generates pulsed light having a pulse width of, for example,50 ns or less. Alternatively, the light source Ls may emit pulsed lighthaving a pulse width of 30 ns or less, or 20 ns or less.

An emission pattern of the light source Ls may have a uniform intensitydistribution, a dotted intensity distribution, or a donut-shapedintensity distribution in an emission area.

1-2. Image Sensor S

The image sensor S receives light emitted from the light source Ls andreflected from the measurement target O. The image sensor S includes aplurality of pixels (light-receiving devices) arranged in two dimensionsand obtains two-dimensional information regarding the measurement targetO at once. The image sensor S, for example, is a charge-coupled device(CCD) image sensor or a complementary metal-oxide-semiconductor (CMOS)image sensor.

The image sensor S includes an electronic shutter. The electronicshutter is a circuit that controls the length (also referred to as the“shutter width”) of a period of one signal accumulation operation inwhich received light is converted into an effective electrical signaland accumulated, that is, the length of an exposure period (alsoreferred to as an “imaging period”), and a time (referred to as a“shutter timing”) taken until an exposure period starts after a previousexposure period ends. In the following description, a state in which theelectronic shutter is open (imaging) will be referred to as an “openstate”, and a state in which the electronic shutter is closed (notimaging) will be referred to as a “closed state”. The image sensor S canadjust the shutter timing on the order of sub-nanoseconds, that is, forexample, 30 ps to 1 ns, using the electronic shutter. A conventionaltime-of-flight (ToF) camera intended for distance measurement detectsall pulsed light emitted from the light source Ls and reflected from asubject in order to correct the brightness of the subject. Theconventional ToF camera, therefore, needs to have a shutter width largerthan the pulse width of the pulsed light. On the other hand, the imagingapparatus D1 according to the present embodiment need not correct theamount of light coming from a subject, and the shutter width need not belarger than the pulse width, that is, may be 1 to 30 ns. According tothe imaging apparatus D1 according to the present embodiment, since theshutter width is small, dark current included in a detected signal canbe reduced.

If the measurement target O is a person's forehead and information suchas a cerebral blood flow is to be detected, an attenuation rate of lightinside the forehead is extremely high (about one-millionth). When theinternal scattering component I2 is detected, therefore, the amount oflight might be insufficient with emission of one pulse. In this case,the light source Ls emits pulsed light a plurality of times, and theimage sensor S accordingly opens the electronic shutter a plurality oftimes. Detected signals are integrated with one another to improvesensitivity.

An example of the configuration of the image sensor S will be describedhereinafter.

The image sensor S includes a plurality of light detection cells (alsoreferred to as “pixels” herein) arranged on an imaging surface in twodimensions. Each cell includes a light receiving device (e.g., aphotodiode).

FIG. 1D is a diagram illustrating an example of a schematicconfiguration of a pixel 201 of the image sensor S. FIG. 1Dschematically illustrates the configuration of the pixel 201 and doesnot necessarily reflect an actual structure. The pixel 201 includes aphotodiode 203 that performs photoelectric conversion, first to fourthfloating diffusion (FD) layers 204 to 207, which are charge accumulationunits that accumulate signal charge, and a drain 202, which is a signalcharge discharge unit that discharges signal charge.

Photons that have entered the pixel 201 as a result of emission of onepulse are converted by the photodiode 203 into signal electrons, whichare signal charge. The signal electrons obtained as a result of theconversion are discharged to the drain 202 or distributed among thefirst to fourth FD layers 204 to 207 in accordance with a control signalinput from the control circuit Sy.

The emission of pulsed light from the light source Ls, the accumulationof signal charge in the first FD layer (FD1) 204, the second FD layer(FD2) 205, the third FD layer (FD3) 206, and the fourth FD layer (FD4)207, and the discharge of signal charge to the drain 202 are repeatedlyperformed in this order. This series of operations is performed at highspeed, that is, for example, the series of operations can be performedtens of thousand times to several hundred million times in one frame(e.g., about 1/30 second) of a moving image. The pixel 201 finallygenerates four image signals based on the signal charge accumulated inthe first to fourth FD layers 204 to 207 and outputs the image signals.

The control circuit Sy accumulates signal charge from the photodiode 203in the first FD layer (FD1) 204 a certain period of time after beginningto emit first pulsed light, and accumulates signal charge from thephotodiode 203 in the second FD layer (FD2) 205 the certain period oftime after beginning to emit second pulsed light. As a result,accumulation of signal charge starts with a phase of a trailing edge ofthe first pulsed light and a phase of a trailing edge of the secondpulsed light matched with each other in the first FD layer (FD1) 204 andthe second FD layer (FD2) 205. If, therefore, a distance to themeasurement target O is changed by ±50 cm, for example, the amount ofsignal charge accumulated increases or decreases in both the first FDlayer (FD1) 204 and the second FD layer (FD2) 205. In addition, in orderto estimate the amount of disturbance light and ambient light, signalcharge may be accumulated in the third FD layer (FD3) 206 with the lightsource Ls turned off. By subtracting the amount of signal charge in thethird FD layer (FD3) 206 from the amount of signal charge in the firstFD layer (FD1) 204 or the second FD layer (FD2) 205, a signal from whichdisturbance light and ambient light components are removed can beobtained. Alternatively, third pulsed light may be accumulated in thefourth FD layer (FD4) 207.

The first pulsed light and the second pulsed light may be light havingdifferent wavelengths. By selecting two wavelengths whose absorptionrates relative to the measurement target O are different from eachother, characteristics of the measurement target O can be analyzed. Iflight having a wavelength longer than 805 nm is used as the first pulsedlight and light having a wavelength shorter than 805 nm is used as thesecond pulsed light, for example, the amount of change in theconcentration of oxyhemoglobin and the amount of change in theconcentration of deoxyhemoglobin in a blood flow of the measurementtarget O can be detected.

Although the number of ED layers is four in the present embodiment, thenumber of FD layers may be set to two or more in accordance withpurposes.

FIG. 1E is a diagram illustrating an example of the configuration of theimage sensor S. In FIG. 1E, each area surrounded by a dash-dot-dot linecorresponds to a pixel 201. Although FIG. 1E illustrates only fourpixels arranged in two rows and two columns, a larger number of pixelsare provided in practice. The pixel 201 includes the first to fourth EDlayers 204 to 207. Signals accumulated in the four ED layers 204 to 207are treated as if the signals are ones in four pixels of a common CMOSimage sensor, and output from the image sensor S.

Each pixel 201 includes four signal detection circuits. Each signaldetection circuit includes a source follower transistor (amplifyingtransistor) 309, an ED signal read transistor (row selection transistor)308, and a reset transistor 310. In this example, the reset transistor310 corresponds to the drain 202 illustrated in FIG. 1D, and a pulseinput to a gate of the reset transistor 310 corresponds to theabove-mentioned pulse discharged to the drain 202. Each transistor is afield-effect transistor formed on a semiconductor board, for example,but is not limited to this. As illustrated in FIG. 1E, either an inputterminal or an output terminal (typically a source) of the sourcefollower transistor 309 is connected to either an input terminal or anoutput terminal (typically a drain) of the FD signal read transistor308. A control terminal (gate) of the source follower transistor 309 isconnected to the photodiode 203. Signal charge (holes or electrons)generated by the photodiode 203 is accumulated in the corresponding FDlayer, which is a charge accumulation unit, between the photodiode 203and the source follower transistor 309.

Although not illustrated in FIG. 1E, each of the first to fourth FDlayers 204 to 207 is connected to the photodiode 203, and a switch canbe provided between the photodiode 203 and each of the first to fourthED layers 204 to 207. This switch connects or disconnects the photodiode203 and each of the first to fourth FD layers 204 to 207 in accordancewith a signal accumulation pulse from the control circuit Sy. As aresult, signal change starts to be accumulated or stops beingaccumulated in each of the first to fourth ED layers 204 to 207. Theelectronic shutter in the present embodiment includes a mechanism forperforming such exposure control.

The signal charge accumulated in the first to fourth FD layers 204 to207 is read when a row selection circuit 302 turns on gates of the rowselection transistors 308. At this time, current flowing from a sourcefollower power supply 305 to the source follower transistors 309 andsource follower loads 306 is amplified in accordance with signalpotentials of the first to fourth FD layers 204 to 207. Analog signalscaused by the current and read by vertical signal lines 304 areconverted into digital signal data by analog-to-digital (A/D) conversioncircuits 307, each of which is connected to a corresponding column. Thedigital signal data is read by a column selection circuit 303 for eachcolumn and output from the image sensor S. After reading one row, therow selection circuit 302 and the column selection circuit 303 read anext row, and then sequentially read information regarding signal chargeaccumulated in the first to fourth FD layers 204 to 207 in all the rows.After reading all the signal charge, the control circuit Sy turns on thegates of the reset transistors 310 to reset all the first to fourth FDlayers 204 to 207. Imaging of one frame is thus completed. Similarly,the image sensor S repeats high-speed imaging of a frame in order tocomplete imaging of a series of frames.

Although the CMOS image sensor S has been taken as an example in thepresent embodiment, the imaging device used may be a CCD, a singlephoton counting device, or an amplifying image sensor(electron-multiplying CCD (EMCCD) or intensified CCD (ICCD)), instead.

As illustrated in FIG. 1F, in the present embodiment, emission of thefirst pulsed light and emission of the second pulsed light may alternatea plurality of times in a frame. In doing so, a difference betweentimings at which two types of images are obtained can be reduced, andimaging can be almost simultaneously performed with the first and secondpulsed light even for a moving measurement target O.

1-3. Control Circuit Sy

The control circuit Sy adjusts a difference between a timing at whichthe light source Ls emits pulsed light and a shutter timing of the imagesensor S. The difference will also be referred to as a “phase” or a“phase delay” hereinafter. The timing at which the light source Ls emitspulsed light is a timing at which the pulsed light emitted by the lightsource Ls begins to rise. The control circuit Sy may adjust the phase bychanging the timing at which the light source Ls emits pulsed light orby changing the shutter timing.

The control circuit Sy may be configured to remove an offset componentfrom a signal detected by a light-receiving device of the image sensorS. The offset component is a signal component caused by ambient light ordisturbance light such as sunlight or light from a fluorescent lamp. Ifthe image sensor S detects a signal with the light source Ls notemitting light, that is, with the light source Ls turned off, the offsetcomponent caused by ambient light and disturbance light can beestimated.

The control circuit Sy, for example, can be an integrated circuitincluding a processor such as a central processing unit (CPU) or amicrocomputer and a memory. The control circuit Sy adjusts the timing atwhich the light source Ls emits pulsed light and the shutter timing,estimates the offset component, and removes the offset timing, forexample, by executing programs stored in the memory. The control circuitSy may further include an arithmetic circuit that performs arithmeticprocessing such as image processing. Such an arithmetic circuit isachieved, for example, by a combination of a digital signal processor(DSP), a programmable logic device (PLD) such as a field-programmablegate array (FPGA), or a combination of a CPU or a graphics processingunit (GPU) and computer programs. The control circuit Sy and thearithmetic circuit may be integrated as a single circuit, or may beseparate circuits.

FIG. 1G is a flowchart schematically illustrating an operation performedby the control circuit Sy. Schematically, the control circuit Syperforms the operation illustrated in FIG. 1G, details of which will bedescribed later. First, the control circuit Sy causes the light sourceLs to emit pulsed light for a certain period of time (step 311). At thistime, the electronic shutter of the image sensor S is closed. Thecontrol circuit Sy keeps the electronic shutter closed until a part ofthe pulsed light is reflected from the surface of the measurement targetO and reaches the image sensor S. Next, the control circuit Sy opens theelectronic shutter when another part of the pulsed light is scatteredinside the measurement target O and reaches the image sensor S (stepS12). A certain period of time later, the control circuit Sy closes theelectronic shutter (step S13). Next, the control circuit Sy determineswhether the number of times of signal accumulation performed has reacheda certain value (step S14). If not, the control circuit Sy repeats stepsS11 to S13 until the number of times of signal accumulation performedreaches the certain value. If determining in step S14 that the number oftimes of signal accumulation performed has reached the certain value,the control circuit Sy causes the image sensor S to generate a signalindicating an image based on signal charge accumulated in the FD layers204 to 207 and output the signal (step 315).

As a result of the above operation, a component of light scatteredinside the measurement target O can be sensitively detected. Theemission of pulsed light and the exposure by the electronic shutter neednot necessarily be performed a plurality of times, but may be performedas necessary.

1-4. Modifications

The imaging apparatus D1 may include an imaging optical system thatforms a two-dimensional image of the measurement target O on a lightreceiving surface of the image sensor S. An optical axis of the imagingoptical system is substantially perpendicular to the light receivingsurface of the image sensor S. The imaging optical system may include azoom lens. If a position of the zoom lens changes, the two-dimensionalimage of the measurement target O is magnified or reduced, and theresolution of the two-dimensional image on the image sensor S changes.Even if the measurement target O is distant, therefore, a desired areacan be magnified and closely observed.

In addition, the imaging apparatus D1 may include, between themeasurement target O and the image sensor S, a bandpass filter thatpasses only light in and around a wavelength band of the light emittedfrom the light source Ls. In this case, an effect of a disturbancecomponent such as ambient light can be reduced. The bandpass filter is amulti-layer film filter or an absorption filter. In consideration of ashift of the band due to the temperature of the light source Ls andoblique incident on the bandpass filter, the bandwidth of the bandpassfilter may be 20 to 100 nm.

In addition, the imaging apparatus D1 may include polarizing platesbetween the light source Ls and the measurement target O and between theimage sensor S and the measurement target O. In this case, a polarizingdirection of the polarizing plate for the light source Ls and that ofthe polarizing plate for the image sensor S are crossed Nicols. As aresult, it is possible to prevent a regular reflection component (acomponent whose incident angle and reflection angle are the same) in thesurface reflection component I1 of the measurement target O fromreaching the image sensor S. That is, the amount of light of the surfacereflection component I1 that reaches the image sensor S can be reduced.

2. Operation

The imaging apparatus D1 in the present disclosure distinguishes theinternal scattering component I2 from the surface reflection componentI1. If the measurement target O is a person's forehead, the signalstrength of the internal scattering component I2 to be detected isextremely low. This is because, as described above, an extremely smallamount of light that satisfies the laser safety standard is emitted andmost of the light is scattered or absorbed by the scalp, cerebral fluid,gray matter, white matter, and blood flow. Furthermore, a change insignal strength due to a change in the volume of blood flow orcomponents in the blood flow while the brain is active is one severaltenth of the total signal strength, that is, extremely small. Imaging istherefore performed while eliminating the surface reflection componentI1, which is several thousand to several ten thousand times larger thanthe signal component to be detected, as much as possible.

The operation of the imaging apparatus D1 according to the presentembodiment will be described hereinafter.

As illustrated in FIG. 1A, when the light source Ls emits pulsed lightto the measurement target O, the surface reflection component I1 and theinternal scattering component I2 are caused. Part of the surfacereflection component I1 and the internal scattering component I2 reachesthe image sensor S. Since, after being emitted from the light source Ls,the internal scattering component I2 passes through the measurementtarget O before reaching the image sensor 5, an optical path of theinternal scattering component I2 is longer than that of the surfacereflection component I1. The internal scattering component I2,therefore, takes more time to reach the image sensor S than the surfacereflection component I1. FIG. 2 is a diagram illustrating opticalsignals that reach the image sensor S after the light source Ls emitsrectangular pulsed light and the measurement target O reflects thepulsed light. A horizontal axis represents time (t) in (a) to (d) ofFIG. 2, and a vertical axis represents intensity in (a) to (c) of FIG. 2and the open or closed state of the electronic shutter in (d) of FIG. 2.The surface reflection component I1 is illustrated in (a) of FIG. 2. Theinternal scattering component I2 is illustrated in (b) of FIG. 2. Acombined component of the surface reflection component I1 illustrated in(a) of FIG. 2 and the internal scattering component I2 illustrated in(b) of FIG. 2 is illustrated in (c) of FIG. 2. As illustrated in (a) ofFIG. 2, the surface reflection component I1 is rectangular. On the otherhand, as illustrated in (b) of FIG. 2, since the internal scatteringcomponent I2 is a combination of light beams that have passed alongvarious optical paths, the internal scattering component I2 has a “tail”at a trailing edge of the pulsed light (a fall time is longer than thatof the surface reflection component I1). In order to extract theinternal scattering component I2 from the optical signal illustrated in(c) of FIG. 2, the electronic shutter is opened after a trailing edge ofthe surface reflection component I1 (when or after the surfacereflection component I1 falls) as illustrated in (d) of FIG. 2. Thisshutter timing is adjusted by the control circuit Sy. As illustratedabove, since it is only required that the imaging apparatus D1 in thepresent disclosure be able to distinguish the internal scatteringcomponent I2 from the surface reflection component I1, the pulse widthand the shutter width may be arbitrarily determined. As a result, unlikethe method in an example of the related art in which a streak camera isused, the imaging apparatus D1 in the present disclosure can be achievedwith a simple configuration, and cost can be significantly reduced.

If the measurement target O does not have a flat surface, a time atwhich light reaches is different between the pixels of the image sensorS. In this case, the shutter timing illustrated in (d) of FIG. 2 may bedetermined in accordance with pixels corresponding to a target area ofthe measurement target O. Alternatively, depth information regarding asurface of the measurement target O may be detected in advance by aknown method, and the shutter timing may be changed for each pixel onthe basis of the depth information. The depth information is positionalinformation in a z direction when a direction of the optical axis of theimaging optical system is denoted by z. An optimal shutter timing forextracting the internal scattering component I2 can thus be set at eachposition in accordance with curves of the surface of the measurementtarget O.

In (a) of FIG. 2, the trailing edge of the surface reflection componentI1 falls vertically. In other words, a time taken for the surfacereflection component I1 to finish falling is zero. In practice, however,the pulsed light emitted by the light source Ls might not be perfectlyvertical, or the surface of the measurement target O might be slightlyuneven. In this case, the trailing edge of the surface reflectioncomponent I1 might not fall vertically. In addition, because themeasurement target O is usually opaque, the amount of light of thesurface reflection component I1 is significantly larger than that of theinternal scattering component I2. Even if the trailing edge of thesurface reflection component I1 only slightly fails to fall vertically,therefore, the internal scattering component I2 undesirably gets buriedin the surface reflection component I1. In addition, ideal binaryreading illustrated in (d) of FIG. 2 might not be achieved due to adelay according to movement of electrons during a reading period of theelectronic shutter. The control circuit Sy, therefore, may delay theshutter timing of the electronic shutter from a point in timeimmediately after the fall of the surface reflection component I1. Thecontrol circuit Sy, for example, may delay the shutter timing by 0.5 to5 ns. In addition, in order to obtain only information regardingportions deeper than a desired depth of the measurement target O, thecontrol circuit Sy may further delay the shutter timing of theelectronic shutter. Instead of adjusting the shutter timing of theelectronic shutter, the control circuit Sy may adjust the timing atwhich the light source Ls emits pulsed light. The control circuit Sy mayadjust a difference between the shutter timing of the electronic shutterand the timing at which the light source Ls emits pulsed light. When achange in the volume of the blood flow or the components in the bloodflow while the brain is active is measured in a noncontact manner, theinternal scattering component I2 undesirably further decreases if theshutter timing is too late. The shutter timing, therefore, may stayaround the trailing edge of the surface reflection component I1.

The light source Ls may emit pulsed light a plurality of times and theelectronic shutter may be opened a plurality of timings such that thesame phase is maintained for the pulsed light, in order to amplify theamount of light of the internal scattering component I2 detected.

Instead of, or in addition to, providing a bandpass filter between themeasurement target O and the image sensor 5, the control circuit Sy mayperform imaging with the light source Ls turned off and the exposureperiod remaining the same in order to estimate the offset component. Theestimated offset component is differentially removed from signalsdetected by the light receiving devices of the image sensor S. A darkcurrent component caused in the image sensor S can thus be removed.

As described above, in the imaging apparatus D1 in the presentdisclosure, the control circuit Sy causes the light source Ls to emitpulsed light and closes the electronic shutter in a period in which apart of the pulsed light is reflected from the surface of themeasurement target O and reaches the image sensor S. On the other hand,the control circuit Sy opens the electronic shutter in a period in whichanother part of the pulsed light is scattered inside the measurementtarget O and reaches the image sensor S. As a result, internalinformation regarding the measurement target O can be obtained whilesuppressing noise caused by a component reflected from the surface. Atime at which the electronic shutter is opened may be a time at which atrailing edge of the pulsed light reflected from the surface of themeasurement target O reaches the image sensor S or later. The internalinformation regarding the measurement target O can thus be obtainedalmost without the noise caused by the component reflected from thesurface.

Second Embodiment

A second embodiment is different from the first embodiment in that thecontrol circuit Sy determines a phase of a shutter timing. Detaileddescription of features common to the second embodiment and the firstembodiment is omitted herein.

A time taken until light emitted from the light source Ls returns to theimage sensor S after being reflected from the measurement target Odepends on a traveling distance of the light. The phase of a shuttertiming, therefore, is adjusted in accordance with a distance between theimaging apparatus D1 and the measurement target O. FIG. 3A is a diagramillustrating an example of the adjustment of a shutter timing. A timeresponse waveform of an optical signal that reaches the image sensor Sis illustrated in (a) of FIG. 3A. The optical signal illustrated in (a)of FIG. 3A is a signal including both the surface reflection componentI1 and the internal scattering component I2. A shutter timing (t=t1)sufficiently later than a time at which the optical signal illustratedin (a) of FIG. 3A begins to fall is illustrated in (b) of FIG. 3A. Ifthe light source Ls emits pulsed light a plurality of times, a phase tbetween the fall of the optical signal illustrated in (a) of FIG. 3A anda next rise of the optical signal is set. At this time, an opticalsignal from the measurement target O is not detected at all, or only asmall optical signal I at a trailing edge is detected. This opticalsignal I has a relatively long optical path, that is, includes arelatively large amount of information regarding a deeper portion of themeasurement target O. A shutter timing (t=t2) earlier than thatillustrated in (b) of FIG. 3A by a time Δt is illustrated in (c) of FIG.3A. In this case, the amount of light of the optical signal I detectedincreases in accordance with the advance in the shutter timing. Shuttertimings (t=t3 and t=t4, respectively, and t3>t4) even earlier than thatillustrated in (c) of FIG. 3A are illustrated in (d) and (e) of FIG. 3A.The shutter timing is thus gradually advanced. A point in time at whichthe optical signal I detected by the image sensor S begins to increasesharply corresponds to a trailing edge of the surface reflectioncomponent I1. If the shutter timing comes immediately before the sharpincrease, therefore, a signal mainly composed of the internal scatteringcomponent I2 can be detected. The time Δt, by which the shutter timingis advanced, is shorter than an extension (fall time) of a trailing edgeof the internal scattering component I2. The time Δt, for example, is 30ps to 1 ns. FIG. 3B illustrates a relationship between pulses of lightemitted from the light source Ls, optical signals that reach the imagesensor 5, and shutter timings. The light source Ls is periodicallyemitting pulsed light. Although the light source Ls emits pulsed lightafter the shutter closes, a period for which the shutter remains openand a period for which the light source Ls emits pulsed light mayoverlap with each other. An off period between two consecutive beams ofpulsed light emitted from the light source Ls, for example, may be up tofour times as long as the shutter width, twice as long as the shutterwidth, or 1.5 times as long as the shutter width. The off period refersto a time taken until pulsed light emitted from the light source Lsrises after the pulsed light finishes falling. The off period may be upto four times as long as the pulse width of the pulsed light emittedfrom the light source Ls, twice as long as the pulse width, or 1.5 timesas long as the pulse width. Since, in this case, the image sensor S canbe exposed to more light in unit time, sensitivity improves with a framerate remaining the same. The off period is too short to be employed in aconventional ToF camera because erroneous detection can occur duringdistance measurement.

Methods for finding an optimal phase of the shutter timing includeiterative methods such as a bisection algorithm and a Newton's methodand other numerical calculations, in addition to the method illustratedin FIG. 3A, in which the phase is gradually changed. By one of thesemethods, the number of times of image capture can be decreased, therebyreducing a time taken to find an appropriate phase.

Alternatively, the phase of the shutter timing may be determined afterdirectly measuring the distance to the measurement target O throughtriangulation with a binocular or multiocular camera or measurement of aflight time by a ToF method. A time taken for a trailing edge of thesurface reflection component I1 of pulsed light emitted from the lightsource Ls to reach the image sensor S after the light source Ls emitsthe pulsed light can be estimated on the basis of the measured distance.The control circuit Sy may open the shutter when or after the estimatedtime elapses. In this case, the control circuit Sy includes anarithmetic circuit that calculates the distance between the imagingapparatus D1 and the measurement target O or a value that depends on thedistance.

FIG. 4A is a flowchart illustrating the operation of the imagingapparatus D1 according to the present embodiment. The operation of theimaging apparatus D1 is achieved, for example, when the control circuitSy controls the components by executing a program stored in the memory.

First, the control circuit Sy controls the phase of the shutter timingand captures a plurality of images. The plurality of images are capturedwith different phases of the shutter timing (step S101). That is, thecontrol circuit Sy captures the plurality of images with varyingdifferences between the timing at which the light source Ls emits pulsedlight and the shutter timing of the image sensor S.

Next, the control circuit Sy determines, for example, whether a changerate of the strength of detected signals obtained from the plurality ofimages captured in step S101 has exceeded a certain threshold on thebasis of changes in the strength over time. If determining that thechange rate has exceeded the certain threshold, the control circuit Sydetermines an appropriate phase of the shutter timing for subsequentshutter timings (step S102). The appropriate phase, for example, may bea timing at which the threshold is exceeded, or may be a timing laterthan the foregoing timing by a certain period of time.

Next, the control circuit Sy captures an image of the measurement targetO with the phase of the shutter timing determined in step S102 (stepS103). That is, the control circuit Sy captures an image of themeasurement target O while synchronizing the timing at which the lightsource Ls emits pulsed light and the shutter timing with each other withthe determined time difference. As a result, the imaging apparatus D1can detect an optical signal mainly composed of the internal scatteringcomponent I2. The pulse width of the light source Ls and the shutterwidth used in step S101 may be the same as or different from ones usedin step S103. FIG. 4B is a flowchart illustrating the operation of theimaging apparatus D1 according to an embodiment different from thatillustrated in FIG. 4A.

First, the control circuit Sy measures a distance between the imagingapparatus D1 and the image sensor S (step S201). More specifically, thecontrol circuit Sy measures the distance to the measurement target Othrough measurement of a flight time by a conventional ToF method orusing a multiocular camera that can be included in the imaging apparatusD1.

Next, the control circuit Sy determines the phase of the shutter timingon the basis of the distance measured in step S201 (step S202).

Next, the control circuit Sy captures an image of the measurement targetO with the phase of the shutter timing determined in step S202 (stepS203). That is, the control circuit Sy captures an image of themeasurement target O while synchronizing the timing at which the lightsource Ls emits pulsed light and the shutter timing with each other withthe determined time difference. As a result, the imaging apparatus D1can detect an optical signal mainly composed of the internal scatteringcomponent I2.

Step S101 or S201, for example, may be performed if a user issues acorresponding instruction or if the measurement target O, namely, forexample, a head, is detected in an imaging area.

As described above, the control circuit Sy determines the differencebetween the timing at which the light source Ls begins to emit pulsedlight and the timing at which the electronic shutter opens. Morespecifically, in an example, the control circuit Sy captures a pluralityof images with varying differences between the timing at which the lightsource Ls begins to emit pulsed light and the timing at which theelectronic shutter opens. The control circuit Sy then determines thetime difference on the basis of the strength of an electrical signalgenerated by the image sensor S on the basis of the plurality of images.In another example, the control circuit Sy calculates the distancebetween the image sensor S and the measurement target O and determinesthe time difference on the basis of the calculated distance. As aresult, a shutter timing with which a component reflected from thesurface of the measurement target O is hardly detected by the imagesensor S can be achieved.

Third Embodiment

A third embodiment is different from the first embodiment n that animaging apparatus D2 includes a plurality of light sources Ls. Detaileddescription of features common to the present embodiment and the firstembodiment is omitted herein.

FIG. 5 is a schematic diagram illustrating the imaging apparatus D2according to the third embodiment. The imaging apparatus D2 includeslight sources Ls1 and Ls2. The imaging apparatus D2 also includes anarithmetic circuit Pr.

The light sources Ls1 and Ls2 emit light in different wavelength bands.Absorption and scattering characteristics of the measurement target Ogenerally vary depending on the wavelength, components of themeasurement target O can be analyzed in more detail by detecting thewavelength dependence of an optical signal caused by the internalscattering component I2. When the measurement target O is a biologicaltissue, for example, oxyhemoglobin (HbO₂) absorbs more light thandeoxyhemoglobin (Hb) at a wavelength of 800 nm or higher. On the otherhand, an opposite phenomenon occurs at a wavelength shorter than 800 nm.It is assumed, for example, that the light source Ls1 emits light of awavelength of about 750 nm and the light source Ls2 emits light of awavelength of about 850 nm. In this case, changes in the concentrationof HbO₂ and Hb in a blood flow from initial values can be obtained bymeasuring the light intensity of the internal scattering component I2caused by the light from the light source Ls1 and the light intensity ofthe internal scattering component I2 caused by the light from the lightsource Ls2 and solving resultant simultaneous equations.

The arithmetic circuit Pr calculates changes in the concentration ofHbO₂ and Hb in a blood flow from initial values, for example, by solvingsimultaneous equations using the light intensity of the internalscattering component I2 caused by the light from the light source Ls1and the light intensity of the internal scattering component I2 causedby the light from the light source Ls2. Brain activity can be estimatedon the basis of the obtained changes.

The arithmetic circuit Pr can be achieved, for example, by a DSP, a PLOsuch as an FPGA, or a combination of a CPU or a GPU and a computerprogram. The arithmetic circuit Pr and the control circuit Sy may beintegrated as a single circuit.

The imaging apparatus D2 may include, between the image sensor S and themeasurement target O, a bandpass filter that passes light having thewavelengths of the light emitted from the light sources Ls1 and Ls2.

The control circuit Sy determines a difference between a timing at whichthe light source Ls1 emits light and the shutter timing of the imagesensor S and a difference between a timing at which the light source Ls2emits light and the shutter timing of the image sensor S by the methoddescribed in the second embodiment. The control circuit Sy may adjustthe timing(s) at which the light source Ls1 and/or the light source Ls2emit light, or may adjust the shutter timing of the image sensor S.

Optical paths of the light that has been emitted from the light sourcesLs1 and Ls2, reflected from the measurement target O, and has reachedthe image sensor S may be the same. A distance between the image sensorS and the light source Ls1 and a distance between the image sensor S andthe light source Ls2, therefore, may be the same. The light sources Ls1and Ls2, for example, may be arranged at rotationally symmetricalpositions around the image sensor S.

The imaging apparatus D2 may include two images sensors S, instead. Inthis case, a bandpass filter that selectively passes light having thewavelength of the light emitted from the light source Ls1 may beprovided in front of one of the image sensors S. In addition, a bandpassfilter that selectively passes light having the wavelength of the lightemitted from the light source Ls2 may be provided in front of the otherimage sensor S. In this case, the light sources Ls1 and Ls2 may emitlight at the same time, and images can be captured simultaneously usingthe two image sensors S. If the imaging apparatus D2 includes only oneimage sensor S, images of two wavelengths can be obtained by capturingan image using the light source Ls1 and capturing an image using thelight source Ls2 at different times.

In the case of a measurement target O that indicates more complexspectral characteristics, the measurement target O can be analyzed moreaccurately by increasing the number of wavelengths. As an imaging methodin which the number of wavelengths is larger, a method in which thenumber of light sources is increased in accordance with the number ofwavelengths or another known method may be used.

Fourth Embodiment

In a fourth embodiment, an example will be described in which an imageof a person's head is captured using one of the imaging apparatusesaccording to the first to third embodiments. Detailed description offeatures common to the present embodiment and the first to thirdembodiments is omitted herein.

FIG. 6A illustrates an example in which changes in brain activity aremeasured using the imaging apparatus D2 according to the thirdembodiment (includes the two light sources Ls1 and Ls2). FIG. 6A is adiagram illustrating a detection area M in which the internal scatteringcomponent I2 is detected, (i) the amount of change in the concentrationof HbO₂ in a blood flow from an initial value measured in the detectionarea M, (ii) the amount of change in the concentration of Hb in theblood flow from an initial value measured in the detection area M, and(iii) temporal changes in the sum of the amount of change in theconcentration of HbO₂ in the blood flow and the amount of change in theconcentration of Hb in the blood flow. If a brain activity state of themeasurement target O changes over time between a normal state, aconcentrated state, a relaxed state, and the like, the changes cause thetemporal changes indicated by (i) to (iii). While (i) to (iii) are beingmeasured, the detection area M may remain at the same position asprecisely as possible. This is because the brain activity and theabsorption and scattering coefficients are different between areas ofthe brain. When temporal changes in the brain activity are observed, anemotional state of the measurement target O can be estimated on thebasis of temporal changes in absorbance (or the amount of light that hasparticular wavelengths and reaches the image sensor S without beingabsorbed) even if absolute magnitudes or true values of theconcentration of HbO₂ and Hb in the blood flow are unknown. In addition,two wavelength bands need not necessarily be used for detecting temporalchanges in the brain activity. Only one wavelength band may be used,instead. That is, an imaging apparatus such as that according to thefirst embodiment, which includes only one light source Ls, may be used.In this case, the light source Ls is preferably configured to emit lightof a wavelength of 810 to 880 nm. This is because the absorbance of HbO₂usually changes more largely than that of Hb when the brain activitychanges, and patterns of changes in the brain activity can be found justby measuring a wavelength range in which the absorbance of HbO₂ is high.

FIG. 6B illustrates an example in which measurement is simultaneouslyperformed at a plurality of points in the detection area M. An emissionpattern of the light source Ls, for example, may have a uniformintensity distribution, a dotted intensity distribution, or adonut-shaped intensity distribution in the detection area M. If light ofa uniform intensity distribution is emitted, adjustment of emissionpositions on the measurement target O need not be performed or can besimplified. In addition, since light enters the measurement target Ofrom various angles, the strength of a signal detected by the imagesensor S can be increased. Furthermore, measurement can be performed atarbitrary spatial positions within the emission area. In the case ofpartial emission such as a dotted intensity distribution or adonut-shaped intensity distribution, an effect of the surface reflectioncomponent I1 can be reduced just by removing the detection area M fromthe emission area. In this case, for example, the image sensor S maydetermine whether each pixel in a captured image falls within theemission area. An area including pixels determined to fall withinnon-emission areas may then be determined as a detection area, andmeasurement for detecting the internal scattering component I2 may beperformed. The determination whether each pixel falls within theemission area may be made on the basis of whether the luminance of eachpixel is larger than a certain threshold. Since the intensity of surfacereflection component I1 and that of the internal scattering component I2are largely different from each other, it becomes difficult to measurethe internal scattering component I2 if the emission area, howeversmall, remains in the detection area M. The present method can thereforebe effectively used.

FIG. 7A illustrates an emission area Ia of the light source Ls, and FIG.7B illustrates a two-dimensional distribution of brain activity in thedetection area M. If a cerebral blood flow is measured in a noncontactmanner, the amount of light detected undesirably attenuates inaccordance with a square of the distance between the measurement targetO and the image sensor S. A signal of each pixel detected by the imagesensor S may be complemented by nearby pixels and strengthened. In doingso, the number of pulses of light emitted from the light source Ls canbe decreased while maintaining the S/N ratio, and the frame rateimproves. In measurement of a cerebral blood flow, a change caused by abrain activity in a normal state is measured in order to read a changein brain activity. Since the image sensor S including the pixelsarranged in two dimensions is used in the present disclosure, atwo-dimensional distribution of brain activity can be obtained asillustrated in FIG. 7B. A portion of a brain that is highly active canbe detected on the basis of a relative intensity distribution even ifthe normal state is obtained in advance.

A state in which the detection area M of the image sensor S has changedduring measurement as a result of movement of the measurement target Odue to breathing or the like is illustrated in (b) of FIG. 7B. Because,in general, brain activity distribution does not sharply changes in ashort period of time, the shift of the detection area M can be correctedthrough, for example, pattern matching between frames of the detectedtwo-dimensional distribution. Alternatively, in the case of periodicalmovement caused by breathing or the like, only corresponding frequencycomponents may be extracted and corrected or removed.

Fifth Embodiment

A fifth embodiment illustrates an example in which one of the imagingapparatuses according to the first to third embodiments is used.Detailed description of features common to the present embodiment andthe first to third embodiments is omitted herein.

In the fifth embodiment, an internal structure of the measurement targetO is diagnosed. In order to diagnose the internal structure of themeasurement target O, the effect of the surface reflection component I1is suppressed. In the fifth embodiment, one of the imaging apparatusesaccording to the first to third embodiments is used to diagnose theinternal structure of the measurement target O. As illustrated in FIGS.8A and 8B, if there is a deterioration or a structural change inside themeasurement target O, such as cracks 802 or cavities 804, the opticalpath of the internal scattering component I2 changes because of a changein an index of refraction or interface reflection. If the optical pathof the internal scattering component I2 changes, a time at which theinternal scattering component I2 reaches the image sensor S and thecoherence of light change. In the fifth embodiment, a deterioration inthe measurement target O or a processing state of the measurement targetO is checked by detecting these changes.

FIG. 9A illustrates an example in which information regarding an insideof a fruit is obtained using one of the imaging apparatuses according tothe first to third embodiments. In an example of the related art, thefruit needs to be peeled before measurement in order to eliminate aneffect of skin. The inside of the fruit, however, can be inspected in anondestructive manner using one of the imaging apparatuses according tothe first to third embodiments while eliminating information regardingthe skin. In FIG. 9B, the image sensor S further includes amulti-wavelength detector Sp. The multi-wavelength detector Sp is acamera that obtains a multi-wavelength image. A camera in which a filterthat passes a different wavelength is provided for each pixel or acamera that includes liquid crystal tunable filter or an acousto-opticdevice, for example, may be used as the multi-wavelength detector Sp.The “multi-wavelength”, for example, refers to four to 100 wavelengths.Alternatively, a compressive sensing camera that generates an image(multi-wavelength information) for each of a multiple of wavelengths byperforming a statistical operation on superimposed codedmulti-wavelength information and reconstructing originalmulti-wavelength information. By obtaining the multi-wavelengthinformation, the sugar content, maturity, and deterioration of themeasurement target O can be measured.

Sixth Embodiment

A sixth embodiment is different from the first to third embodiments inthat the internal scattering component I2 of the measurement target O isresolved for each optical length. Detailed description of featurescommon to the present embodiment and the first to third embodiments isomitted herein.

A time response waveform of an optical signal that reaches the imagesensor S is illustrated in (a) of FIG. 10. If the pulse width of thelight source Ls is small enough, it can be regarded that an opticallength distribution is illustrated in (a) of FIG. 10. That is, sincelight having a long optical path reaches the image sensor S late, thetime t at which the light is detected is large (late). That is, the timeresponse waveform has an extension according to the optical lengthdistribution.

Shutter timings with the phase t=t, t2, t3, and t4, respectively(t1>t2>t3>t4) are illustrated in (b) to (e) of FIG. 10. The “phase”refers to a difference between the timing at which the light source Lsemits pulsed light and the shutter timing of the image sensor S. Adiagram in which a horizontal axis represents the phases of the shuttertimings illustrated in (b) to (e) of FIG. 10 and a vertical axisrepresents the strength of a signal I detected by a target pixel at eachshutter timing is illustrated in (f) of FIG. 10. The detected signal Iincludes light having a relatively long optical path when t=t1, which isa latest time, and includes light having a relatively short optical pathwhen t=t4, which an earliest time. An optical length distributioncorresponding to a trailing edge of the optical signal illustrated in(a) of FIG. 10 can be obtained, for example, by differentiating thegraph (f) of FIG. 10 or obtaining differences between detected signals Ifor each phase difference. Because the optical length distributionrelates to length information regarding an inside of the measurementtarget O, depth information regarding the measurement target O can beestimated.

The phase may be changed by changing the shutter timing or by changingthe timing at which the light source Ls emits pulsed light.

If the pulse width of the light source Ls is large, the pulsed light ofthe light source Ls can be regarded as a series of pulsed light beamshaving short pulse widths. The optical signal illustrated in (a) of FIG.10, therefore, is convolution of detected signals caused by the pulsedlight beams having short pulse widths. A desired optical lengthdistribution can be obtained, therefore, by differentiating a derivativeof the graph of FIG. 10(f) (that is, by obtaining a second derivative).

As described above, the control circuit Sy detects a time-resolvedsignal at a time at which a trailing edge of pulsed light reflected fromthe surface of the measurement target O reaches the image sensor S orlater. More specifically, the control circuit Sy captures a plurality ofimages with varying phases. An optical length distribution of theinternal scattering component I2 can be obtained on the basis of changesin signal strength obtained from the plurality of images.

Seventh Embodiment

In a seventh embodiment, an example will be described in which aninternal scattering component I2 distant from the surface of themeasurement target O is detected using one of the imaging apparatusesaccording to the first to third embodiments. Detailed description of thesame features as in the first to third embodiment is omitted.

FIG. 11 is a diagram illustrating an imaging apparatus that detects arelatively large amount of the internal scattering component I2 distantfrom the surface of the measurement target O among components scatteredinside the measurement target O. The internal scattering component I2distant from the surface of the measurement target O can be obtained bymaking the shutter timing of the image sensor S later than a trailingedge of the surface reflection component I1. In the case of a shuttertiming with which the surface reflection component I1 is not included,the shutter width may be sufficiently larger than the pulse width ofemitted pulsed light. The shutter width, for example, may be five, 10,or 100 times as large as the pulse width.

First Example

An example in which an image of a person's forehead was captured as themeasurement target O will be described. In FIG. 12, a horizontal axisrepresents a difference between the shutter timing and the timing atwhich the light source Ls emits pulsed light, that is, a phase delay,and a vertical axis represents the strength of an optical signal I fromthe image sensor S obtained from an image captured at the shuttertiming. During a process for capturing images, the shutter timing waschanged in steps of 200 ps. The detection area M in which opticalsignals I were detected includes 50×50 pixels. The wavelength of lightemitted from the light source Ls was 850 nm. A distance between theforehead and the light source Ls (image sensor 5) was 10 cm. A sheet ofpaper that blocks near-infrared light covered part of the forehead, andmeasurement was performed on both the forehead and the sheet of paper.FIG. 12 illustrates light returning from the forehead and lightreturning from the sheet of paper. Values along the vertical axisillustrated in FIG. 12 are values normalized with pixel valuesseparately obtained by capturing images of the forehead and the sheet ofpaper using the entirety of pulsed light such that saturation does notoccur. In FIG. 12, a part around a phase delay of 1 ns in which strengthsharply drops corresponds to a trailing edge of pulsed light. Asillustrated in FIG. 12, in a part later than the trailing edge of thepulsed light, the light returning from the forehead is more intense thanthe light returning from the pieces of paper. That is, the lightreturning from the forehead includes the internal scattering componentI2 at times later than the trailing edge of the pulsed light. It canthus be seen that only information regarding the person's brain can bedetected in a noncontact manner.

FIG. 13 illustrates a result of capture of images of the forehead at ashutter timing fixed at a phase delay of 4 ns. During the image capture,a subject alternately entered a relaxed state and a concentrated state(calculation) at intervals of 30 seconds. Measurement was performed atintervals of 30 seconds, and results of the measurement were normalizedwith maximum and minimum values. In FIG. 13, the strength of detectedlight signals during calculation are lower than in the relaxed state.This can be because the light of a wavelength of 850 nm is absorbed moreby HbO₂ than by Hb, and HbO₂ that had increased during calculationabsorbed the light.

What is claimed is:
 1. An imaging apparatus comprising: a light source that, in operation, emits pulsed light to a measurement target; a diffusion member that is disposed between the light source and the measurement target, and diffuses the pulsed light; an image sensor that includes at least one pixel, the at least one pixel including a photodiode and a charge accumulator that, in operation, accumulates signal charge from the photodiode; and a control circuit that, in operation, controls the image sensor, wherein the control circuit, in operation; causes the image sensor to start to accumulate the signal charge with the charge accumulator in a falling period of a returned pulsed light which is returned from the measurement target to the image sensor due to the emission of the pulsed light, the falling period being a period from start to end of a decrease of an intensity of the returned pulsed light.
 2. The imaging apparatus according to claim 1, further comprising: an arithmetic circuit that, in operation, obtains internal information regarding the measurement target by calculating the signal charge.
 3. The imaging apparatus according to claim 1, wherein: the image sensor further includes an electronic shutter; the control circuit, in operation, causes the electronic shutter to prevent the charge accumulator from accumulating the signal charge while a part of the pulsed light is reflected from a surface of the measurement target and reaches the image sensor; and the control circuit; in operation, causes the electronic shutter to allow the charge accumulator to accumulate the signal charge while another part of the pulsed light is scattered inside the measurement target and reaches the image sensor.
 4. The imaging apparatus according to claim 1, wherein the at least one pixel comprises pixels arranged in two dimensions.
 5. The imaging apparatus according to claim 3, wherein the control circuit, in operation, causes the electronic shutter to allow the charge accumulator to begin to accumulate the signal charge when a trailing edge of the pulsed light is reflected from the surface of the measurement target and reaches the image sensor or later.
 6. The imaging apparatus according to claim 1, wherein: the control circuit, in operation, causes the charge accumulator to begin to accumulate the signal charge a period of time after the light source begins to emit the pulsed light; and the control circuit, in operation, determines the period of time on the basis of intensity of the signal charge accumulated in the charge accumulator.
 7. The imaging apparatus according to claim 1, wherein: the control circuit, in operation, causes the charge accumulator to begin to accumulate the signal charge a period of time after the light source begins to emit the pulsed light; the control circuit, in operation, calculates a distance between the image sensor and the measurement target; and the control circuit, in operation, determines the period of time on the basis of the distance.
 8. The imaging apparatus according to claim 1, wherein the light source, in operation, emits first pulsed light in a first wavelength band and second pulsed light in a second wavelength band, the second wavelength band being different from the first wavelength band.
 9. The imaging apparatus according to claim 1, wherein the image sensor is configured to obtain a multi-wavelength image.
 10. The imaging apparatus according to claim 1, wherein: the control circuit, in operation, causes the charge accumulator to accumulate the signal charge at a plurality of times when a trailing edge of the pulsed light is reflected from a surface of the measurement target and reaches the image sensor or later, and the control circuit, in operation, obtains an optical length distribution of the pulsed light returned from the measurement target on the basis of a change in intensity of the signal charge accumulated at the plurality of times.
 11. The imaging apparatus according to claim 1, further comprising: a correction circuit that, in operation, corrects movement of the measurement target.
 12. The imaging apparatus according to claim 11, wherein the correction circuit, in operation, corrects the movement of the measurement target by detecting periodic vibration of the measurement target.
 13. The imaging apparatus according to claim 2, wherein: the measurement target is a living body, and the internal information is biological information regarding the living body.
 14. The imaging apparatus according to claim 13, wherein the biological information is a brain activity of the living body.
 15. The imaging apparatus according to claim 2, wherein a first area of the measurement target on which the pulsed light is illuminated is larger than a second area from which the internal information of the measurement target is obtained.
 16. The imaging apparatus according to claim 15, wherein at least a part of the second area overlaps at least a part of the first area.
 17. The imaging apparatus according to claim 1, wherein a pulse width of the pulsed light is 3 ns or more.
 18. The imaging apparatus according to claim 1, wherein the pulse width of the pulsed light is 5 ns or more.
 19. The imaging apparatus according to claim 1, wherein the pulse width of the pulsed light is 10 ns or more.
 20. The imaging apparatus according to claim 1, wherein: the returned pulsed light includes two light pulses that sequentially reach the image sensor, and the control circuit, in operation, causes the image sensor to start to accumulate the signal charge with the charge accumulator in the falling period of each of the two light pulses. 